1. Field of the Invention
This invention relates in general to seismic prospecting, and in particular, it relates to a method for performing seismic surveys.
2. Description of the Prior Art
To locate reflecting interfaces in the earth, seismic signals are transmitted through the earth, reflected by subterranean interfaces and are detected and recorded. The time lapse between the transmission, reflection by an interface, and detection of the signal gives the two-way travel time of the signal through the earth, which is then used to located the interface.
It has become common to use seismic vibrators in seismic prospecting. Typically, a number of vibrators emit a long swept frequency signal in the seismic frequency range. The emitted signal (after reflection and refraction within the earth) is received by detectors spaced along a spread, and the received signal is cross-correlated with the emitted swept frequency signal. This operation yields a seismic record which is then processed into a representation of a cross-section of the earth, using techniques well known in the art.
Typically, three vibrators may be used in a given seismic survey in order to increase the energy transmitted to the earth. All three vibrators are positioned at substantially the same location and are controlled to emit simultaneously the same signal. Although three vibrators may be used, only one survey is conducted. In many instances, it would be economical to conduct a plurality of surveys simultaneouly, especially when performing three-dimensional surveys. However, in order to conduct multiple surveys at the same time, at the same general location, it is necessary somehow to be able to distinguish, in the recorded signal, signals being emitted from one vibratory source from the signals emitted by the other vibratory sources. The term "vibratory source" is intended to means either a single vibrator or a plurality of vibrators which are controlled to emit a common signal at substantially the same location.
In U.S. Pat. No. 3,885,225, issued May 20, 1975, to Anstey et al, there is shown one method of conducting a plurality of surveys simultaneously. In this method, the swept frequency seismic signal is broken up into a number of component parts. If, for example, it is desired to emit signals within the frequency range between 10 and 46 Hz, this range may be broken into three segments of 10 to 22 Hz, 22 to 34 Hz, and 34 to 46 Hz. Three different vibratory sources are utilized, each one simultaneously emitting signals from a different one of these frequency segments, so that if the first vibrator is emitting a signal in the range of 10 to 22 Hz, the second vibrator might be emitting energy in the range of 22 to 34 Hz, and the third vibrator emitting energy in the range of 34 to 46 Hz. Reflected energy from all three sources is detected by the same geophones and recorded. However, when this recorded signal is cross-correlated with the signal emitted by the first vibrator, which extends only from 10 to 22 Hz, the signals originating from the second and third vibrators in the ranges of 22 to 34 Hz, and 34 to 46 Hz, will be substantially eliminated from the cross-correlated record. The received signal is similarly cross-correlated with the signal emitted by the second vibrator between 22 to 34 Hz, and with the signal emitted by the third vibrator between 34 and 46 Hz.
After the initial frequency segments are injected into the earth, the control signals for all three vibratory sources are interchanged so that, for example, vibrator source number 1 now emits energy in the 22-34 Hz range, vibrator 3 emits energy in the 10-22 Hz range and vibrator 1 emits energy in the 22-34 Hz range. Subsequently, another sweep is made in which vibrator 1 now emits energy in the 34-46 Hz range, vibrator 2 emits energy in the 10-22 Hz range, and vibrator 3 emits energy in the 22-34 Hz range. In each instance, the recorded signal is correlated separately with the transmitted signal from each of the vibrators to discriminate against the energy emitted by the other vibrators so that after three emissions, a cross-correlated record of energy extending over the entire desired frequency range of 10 to 46 Hz will be attained for each of the three vibrators. The three cross-correlated records corresponding to each vibrator are then stacked. A disadvantage of this method is that three separate sweeps must be made to generate signals over the desired frequency range. Because a "listening" time is required after each sweep to allow energy to return to the surface from the deepest formations of interest, use of this method is somewhat slower than would be the case if each vibrator could be swept through the entire frequency range of interest without stopping.
U.S. Pat. No. 4,295,213, issued Oct. 13, 1981 to Mifsud, also discloses the possibility of transmitting simultaneously a plurality of seismic signals extending over different frequency range. This disclosure, however, was directed towards using alternate upsweeps and downsweeps for different segments of the frequency band in the conducting of a single survey. The invention is directed toward utilizing a segmented sweep as a means of reducing correlation noise within a single survey rather than enabling a plurality of surveys to be conducted simultaneously.
In European Patent Application, Publication No. 0122824, and corresponding Australian Patent Application, No. AU-A-25893/84, there is disclosed a method of simultaneously conducting two surveys. In this method, two vibratory sources simultaneously emit separate vibratory signals. The first of these vibrators successively transmits an identical signal. The second vibrator also successively transmits a signal which is identical during each transmission, except that during alternate transmissions, the phase of the signal is reversed. After the received signals from these transmissions are recorded, successive pairs of these received signals are summed together. It can be seen that when two such recorded signals are summed together, the energy emitted by the first vibratory source will sum together, but since the energy emitted by the second vibratory source has an alternate phase reversal, the energy from the second source will be substantially omitted from the record. A second summation of the two signals is also made, but prior to making the second summation the second recorded signal of the pair is reversed in phase. The result now is that signals originated from the first vibratory source are in opposite phase, and signals originating from the second vibratory source are now in phase. After summation of this signal pair, the signal from the first vibratory source will be substantially eliminated and the signals from the second source will be added. Such a straightforward approach, of course, is usable with only two vibratory sources.
Alternatively, the two vibratory sources may continually transmit the same signal, with one vibrator sweeping from a low frequency to a high frequency and the other source sweeping from a high frequency to a low frequency. Use of this method, however, produces cross-talk between the two transmitted signals. See also "Simultaneous Recording of Several Vibroseis Seismic Lines," by R. Garotta, 1983 SEG Abstracts, and "Simultaneous Recording of Several Vibroseis Seismic Lines" CGG Technical Series No. 531.83.07.
The use of pseudo-random codes to perform multiple seismic surveys simultaneously has also been proposed. See "Signal Design in the "VIBROSEIS.RTM. Technique" by Pierre L. Goupillaud, presented at the 44th Annual International SEG meeting, Nov. 12, 1974. In this method, a set of pseudorandom signals which are substantially orthogonal (uncorrelated) each to the other are transmitted by the different sources. When the recorded signal, which includes signals emitted by each of the sources, is correlated with a selected one of the transmitted signals, the signals from the other sources will be substantially eliminated. It is difficult to generate such signals with seismic vibrators, however. In general vibrators are capable of transmitting only sine waves, which may be "swept" with time.